Significant figures correspond to the number of digits that a number contains. Zeros at the beginning and end of the number are not counted, only zeros are counted if they are in an intermediate position.
For this case, the number is written in scientific notation, the corresponding 10 of the scientific notation is not taken into account during the digit count, therefore the significant figures of this number will be:
Answer: So, in the number 4.150 x10^-4, there are 3 significant figures